WL#2731: Multi-source: SHOW SLAVE STATUS

SUMMARY
1. Add command: SHOW SLAVE STATUS FOR ALL MASTERS
This was planned as part of WL#1697, but is not included
in the current patch.
2. For SHOW SLAVE STATUS, retried_trans should be reported.
(In the current patch, the retried_trans variable shows
the sum of all retired trans for all named masters, in
SHOW SLAVE STATUS it should be possible to get this
value for a single one.)